New methods were developed and applied both to obtain very precise
x-ray lattice parameters along the melting curves of solid argon and solid
krypton and to measure changes in vacancy concentrations along quasi-isochores for both these substances. The uncertainty of the lattice parameters obtained along melting was estimated to be 27 ppm. These measurements are believed to be the most precise melting line density data ever reported for argon and krypton. The uncertainty of expansion measurements off melting was estimated to be 7 ppm, and this implies a sensitivity of these data of 2.1 x 10-5 to changes in the concentration of vacancies. Five separate argon crystals yielded lattice parameters along the argon melting curve over the temperature range 86.7 K to 124.3 K. This corresponded to the molar volume range 23.43 cm3/mole to 24.54 cm3/mole.. Vacancy information was obtained from three of these crystals. A krypton crystal yielded melting curve lattice parameters over the temperature range 127.3 K to 138.9 K which corresponded to the molar volume range 29.32 cm3/mo1e to
29.62 cm3/mo1e. These constitute the first reported melting densities for solid krypton in the range given. The krypton crystal also yielded vacancy information. The temperature region just below melting was investigated in some detail for one argon crystal and the krypton crystal, and ananoma10usly rapid variation in lattice parameter within a degree or so of melting was observed in both.
With the very precise lattice parameters at melting, are within about 1 cm3/mo1e of the molar volume at the triple point can be obtained for both argon and krypton with a precision at least five times better
than any value reported previously. The vacancy measurements in krypton indicate that vacancy concentrations are constant to within the sensitivity along isochores of krypton. This result together with values for the energy of formation of a vacancy at the saturated vapor pressure and knowledge of the equation of state yields values for vp' the volume of formation of a vacancy. The vacancy measurements in solid argon were confused by an apparent flow of material due to small changes in volume of the sample cell. A description of this process is given and a value for vp in argon is extracted.